Generally, in an air conditioner for a vehicle, a drain pipe, refrigerant pipes and hot water pipes are arranged at the vehicle front side of an interior air conditioning unit (air conditioning unit) mounted inside a passenger compartment, to improve a mounting performance of the air conditioning unit, for example, referring to JP-2003-72360A. The refrigerant pipes are used for a refrigerant cycle through an evaporator of the air conditioning unit. Water (drain water) condensed in the evaporator is discharged out of the air conditioning unit through the drain pipe. The hot water pipes are used for a hot water cycle through a heater core of the air conditioning unit.
In the vehicle, the passenger compartment is separated from an engine cabin by an isolation member (i.e., dashboard). The isolation member is generally provided with a pipe penetration opening, through which the refrigerant pipes, the hot water pipes and the drain pipe extend to the air conditioning unit from the engine cabin (outside passenger compartment). Therefore, water (e.g., vehicle-washing water or water from road) may enter the passenger compartment from the side of the engine cabin through the pipe penetration opening. However, a seal structure for restricting water from entering the passenger compartment through the pipe penetration opening is not detailed in JP-2003-72360A.
FIG. 8 shows a seal structure described in other related art. In this case, a pipe penetration opening 43 is provided for an isolation member 40, which isolates a passenger compartment 42 from an engine cabin 41 disposed at the vehicle front side. A refrigerant pipe 30 extends into the passenger compartment 42 from the engine cabin 41 through the pipe penetration opening 43.
The interior end (inside passenger compartment 42) of the refrigerant pipe 30 is connected to one end of a refrigerant passage of an expansion valve 23 through a block joint 24 (pipe coupler). The other end of the refrigerant passage is connected to a refrigerant passage of an evaporator (not shown), which is a cooling heat exchanger in an interior air conditioning unit 10 (air conditioning unit 10).
The air conditioning unit 10 is mounted inside an instrument panel disposed at the most front portion of the passenger compartment 42. A pipe-coupling opening wall 15 (pipe coupling wall 15) is formed at the front end of a case 13h of the air conditioning unit 10. The pipe coupling wall 15 is arranged at an annular shape to have an opening facing the pipe penetration opening 43 of the isolation member 40. The block joint 24 is arranged at the inner side of the pipe coupling wall 15, which is located at the vehicle front side with respect to the expansion valve 23. The front-rear direction and the up-down direction indicated in FIG. 8 respectively correspond to the longitudinal direction and the height direction of the vehicle.
In this case, water (e.g., rain water or vehicle-washing water) may enter the passenger compartment 42 from the engine cabin 41 through the pipe penetration opening 43, as indicated by the arrow ‘a’ in FIG. 8.
The seal structure for preventing water from leaking into the passenger compartment 42 is constructed of a first, second and third packing units 51, 52, 53, all of which have an annular shape. The first and second packing unit 51, 52 are wrapped around the block joint 24, while the second packing unit 52 is disposed at the vehicle front side of the first packing unit 51. The rear end surface (bond portion 52a) of the second packing unit 52 is bonded to the front end surface of the first packing unit 51. The whole inner surface (bond portion 51a) of the first packing unit 51 is bonded to the outer surface (metal surface) of the block joint 24.
An outer wall 16 is further formed at the front end of the case 13h. The outer wall 16 is arranged at the outer side of the pipe coupling wall 15 and separated from it at a predetermined distance.
The third annular-shape packing unit 53 is located between the isolation member 40 and the front ends of the walls 15, 16, and arranged along the rim portion of the pipe penetration opening 43. The rear end surface of the third packing unit 53 is bonded to the front end surface of the walls 15, 16 to construct bond portions 53a, 53b. 
In this case, the walls 15 and 16 are arranged at a double-ring shape to form a space 19 therebetween. Thus, the whole case 13h (made of resin, for example) including the two walls 15 and 16 can be formed to have a homogeneous thickness, to prevent a resin formation defect generated in a large-thickness part.
However, the seal structure including the first, second and third packing units 51-53 cannot effectively restrict water from entering the passenger compartment 42, as described in following.
According to this related art, the third packing unit 53 is only bonded to the front end surfaces of the walls 15 and 16 which are arranged at the double-ring shape, to have a small bond area. Therefore, a gap between the third packing unit 53 and the walls 15 and 16 is easily generated due to a faulty bond therebetween. Then, water which enters the rear side of the third packing unit 53 as indicated by the arrow ‘a’ may leak into the passenger compartment 42 through the gap as indicated by the arrow ‘b’.
Moreover, the second packing unit 52 is only bonded to the front end surface of the first packing unit 51. When water enters the case 13h at a high speed (as indicated by arrow ‘a’), for example, in a vehicle high-speed traveling or a washing of the vehicle, the inner surface of the second packing unit 52 will curl up due to a high-speed inflow of water so that water enters the inner side of the second packing unit 52.
Water entering the inner side of the second packing unit 52 will accumulate at the lower side of the inner surface of the pipe coupling wall 15 due to the self weight, to directly contact the first packing unit 51. In this case, the first packing unit 51 is originally set not to directly contact water, and made of a cheaper material (e.g., polyether urethan foam) having a lower watertightness capacity, as compared with that of the second packing unit 52. Then, water will enter the case 13h through a minute gap between the outer surface of the first packing unit 51 and the inner surface of the pipe coupling wall 15, as indicated by the arrow ‘c’.
Water entering the case 13h will be discharged out of the passenger compartment 42 through a drain-water outlet (not shown) mounted at the bottom portion of the case 13h. Because the case 13h (made of resin) is generally divided into two parts in the vehicle left-right direction by a division member, water having entered the case 13h may be leaked into the passenger compartment 42 along the division member and flow into a driving motor of a blower to cause a failure of the driving motor.
Furthermore, in this case, the first packing unit 51 is only fixed between the block joint 24 and the pipe coupling wall 15, without being supported by the case 13h. That is, the rear end surface of the first packing unit 51 directly faces a space 54.
Then, the first packing unit 51 pushed by water in the direction indicated as the arrow ‘a’ will move to the vehicle rear side. As a result, the minute gap between the outer surface of the first packing unit 51 and the inner surface of the pipe coupling wall 15 is enlarged. Therefore, water entering the case 13h is further increased.